Electric machines normally have a rotor and a stator which can be structured in different ways. For instance, magnetic components in the stator or in the rotor can have uniformly configured skewing. For example, the prior art describes a continuous, stepless skewing in the stator by means of a transposition of a laminated core, which, however, requires a laborious winding process, or else it describes a continuous skewing in the rotor, for example, by means of a skewed magnetization of a magnet ring.
Moreover, magnets in the rotor can form uniformly stepped skewing. Such an arrangement can be attained by means of several identical rotor segments that contain permanent magnets and that are twisted relative to each other, whereby the twist angles between adjacent rotor segments are of the same magnitude. Normally, more than two rotor segments are employed as an approximation to the continuous skewing.
During the operation of an electric machine that has a rotor without skewing, a cogging torque is generated that causes fluctuations in the torque of the electric machine. During operation of the rotor, the mechanical twisting of the rotor segments relative to each other causes cogging torques of the appertaining rotor segments which are phase-shifted and superimposed on each other. The total cogging torque can thus be reduced. A suitable cumulative angle of the twist angles can even bring about an extinction of the basic order of the cogging torque and influences the torque ripple under load.
German patent document DE 10 2005 015 657 and German patent application DE 41 12 906 A1 disclose examples of a rotor comprising two or more rotor segments that contain permanent magnets. The rotor segments are arranged so that they can be twisted relative to each other in such a way that, during operation, twisting makes it possible to change between a uniformly stepped arrangement and a rectilinear arrangement of the permanent magnets of the rotor.
During the operation of prior-art electric machines, acoustic noises occur that are caused by a torque ripple. These noises can be perceived as annoying by vehicle occupants. Therefore, these electric machines do not adequately meet the ever-increasing requirements being made in terms of passenger comfort.